This invention relates to marking or writing instruments and, more particularly, to ink reservoir elements for use in such instruments.
Ink reservoir elements for use in marking and writing instruments have conventionally been formed of a fibrous bundle compacted together into a rod-shaped unit having longitudinal capillary passageways which extend therethrough between the fibers and which serve to hold the ink and release it at the required controlled rate. For a number of years, the fibrous material generally employed was cellulose acetate fibers, which could readily be heat-bonded together with suitable plasticizers into a unitary body, and which were compatible with all of the ink formulations then in use. In recent years, however, ink formulations became more sophisticated so that the writing instruments did not need to be capped to prevent the ink from evaporating. Such new ink formulations required formic acid, which was not compatible with cellulose acetate. For this reason, various thermoplastic fibers and, in particular, polyester fibers, had to be used in place of the cellulose acetate fibers for producing the ink reservoir elements.
Various problems have been encountered in attempting to bond polyester fibers together into a unitary ink reservoir body. When adhesives have been employed in the bonding operation, such adhesives have interfered with the capillary action and ink absorption of the units. Efforts made to heat-bond the polyester fibers to each other without any additive adhesive have not met with much success. Because of the narrow softening point of polyester, it has not been possible to heat-bond drawn polyester fibers such as tow. Undrawn polyester fibers could be heat-bonded together, but produced an unusable product because of shrinkage during processing and lack of stability in the presence of inks at the temperature required for storage of writing instruments. Consequently, polyester fiber ink reservoir elements have heretofore been commercially produced in the form of an unbonded bundle of fibers compacted and held together in a rod-shaped unit by means of a porous film overwrap, and generally including a small diameter plastic "breather" tube disposed between the fibrous bundle and the overwrap and serving as an air release passage. Sometimes, the design of the writing instrument barrel precludes the necessity of a separate "breather" tube.
The film-overwrapped polyester fiber ink reservoir elements, when made with parallel continuous-filament fibers, have had adequate ink holding capacity and ink release properties for use with certain types of marking or writing instruments, for example, those employing fiber tips. However, they have not been successful with the more recent roller marker type of writing instrument, due to the fact that the roller markers require a faster ink release then the conventional fiber tips. Efforts to lower the fiber density and/or change the fiber size to increase the ink release have had limited success because the release is not uniform from start to finish. Also, lowering the fiber density has been found to reduce the ink holding capacity of the reservoir. Forming the reservoir from staple fibers randomly laid, rather than from continuous-filament parallel fibers, has been found to increase the ink release properties of short-length reservoirs, but at the longer lengths required for adequate ink holding capacity, this construction lacks the capillarity to function. Thus, it has not previously been possible to form a polyester fiber ink reservoir element having the proper combination of ink holding capacity and ink release properties satisfactory for use in the roller marker type of writing instrument.